Maximum measured resistance on LDR is 5.2Kohms, resulting virtually in a 10K pot.

Managed to increase resistence of every LDR from around 5K in two ways.
One way was to decrease voltage by installing in series with 5V reg one 1n4001 diode and result was around 6.5K not so close to targeted ~9-10K

Better result was putting in series a 50R resistence. In this way a have measured around 10K maximum resistence.

Guys
I think that many if not all of the current sources in this power point presentation could be implemented to work with Lightspeed like circuit.http://www.google.com/url?sa=t&rct=j...56643336,d.b2I
They start on slide 18.
Like slide 19 which simply needs a variable resistor for Ri and then the LDR is the load resistor in the feedback loop. Obviously you use a higher voltage.
If you give the opamp a real nice supply and then use a super quiet reference voltage in place of the 1V shown I think you might have a super easy really nice performing control for the LDRs. You would use one of these circuits for both shunt and one of these circuits for both series. Like in the Lightspeed circuit you would use opposite sides of the wiper of the pot and in this case the two gangs would not be connected to each other.
Check it out. Pretty easy to build if you have an opamp, a few 9V batteries and a few pots.

Guys
I think that many if not all of the current sources in this power point presentation could be implemented to work with Lightspeed like circuit.http://www.google.com/url?sa=t&rct=j...56643336,d.b2I
They start on slide 18.
Like slide 19 which simply needs a variable resistor for Ri and then the LDR is the load resistor in the feedback loop. Obviously you use a higher voltage.
If you give the opamp a real nice supply and then use a super quiet reference voltage in place of the 1V shown I think you might have a super easy really nice performing control for the LDRs. You would use one of these circuits for both shunt and one of these circuits for both series. Like in the Lightspeed circuit you would use opposite sides of the wiper of the pot and in this case the two gangs would not be connected to each other.
Check it out. Pretty easy to build if you have an opamp, a few 9V batteries and a few pots.

I have been playing with circuits like these for quite a while, and I agree that current control rather than voltage control is the answer to really tight control of LDRs, but there are challenges that must be overcome, and it's not so easy. Figure 8-21 is the best circuit for LDR control, but if you build it, you must consider the following:

1. The four resistors must be very close tolerance, *better* than 1%

2. The four resistors must be thermally connected or thermally compensated because any deviation from perfectly matched will make a big difference in the accuracy of the circuit and will cause the control to wander.

3. The resistors must be of fairly low value, and that gives the circuit a low input impedance, and that means you cannot drive it with a simple pot -- it requires a control circuit that can accurately drive a low impedance input which typically means it must be driven by an opamp buffer.

4. The circuit must be capable of accurately controlling a very large range of current from 10 ma down to about .01 ma in order to control an LDR between 50 ohms and 10K ohms, and that is not easy to achieve.

However, I do believe that this is the best wasy to control LDRs, and I have been able to solve these issues, so it's doable.

For a few years now I have been working off-and-on (mostly 'off') towards designing a PIC-controlled passive LDR volume control. The hardware (which I enjoy working on) has been ready for a long time, but the software part (which is not my forte) has lagged.

But over the past week I've really applied myself and have made a lot of progress. I've arrived at a point where I need to make some decisions regarding building flexibility into selecting the potentiometer curve. A conventional potentiometer would have a log response and the sum of R1 and R2 would always equal the nominal value of the pot. For example, to create a 10K pot, R1 plus R2 would always equal 10K.

However, My pot doesn't have to be limited in that way. I could, for example, create a pot where R1 (series) varies between 50 ohms and 15K ohms, and R2 (shunt) varied between 50 ohms and 10K ohms. Or any other variation along those lines.

My control design will allow practical pots with values between maybe 1K and 50K ohms, and a wide range of differing max R1 and max R2 values. I'm thinking a 10K pot with a greater than 10K series resistor would be most useful for a solid state system, but if I've got it wrong I'd be interested in hearing other opinions.

Hi Wapo
I've built an Arduino-controlled LDR volume control: Sylonex and Arduino preamp. It works great, and the resolution is fine. A drawback is that the circuit is rather complex. An advantage is that carefull matching of the LDRs is not needed.
In my opinion input-resistance of a circuit needs to remain 100% stable to avoid influence on the source before. This means you can calculate the values you need at each position of the pot, taking into account the resistance of the input behind the volume control which comes in parallel with the resistance to ground.
I didn't do this as I have a DCB1 behind the volume, which has a very high input resistance, so I keep the sum of series and ground resistor constant at 50kR. I prefer a relatively high input impedance to avoid upsetting my DAC (a TDA1543 8xparallel).
It's easy to change the total resistance in soft, but I haven't written this yet. It could be better to reduce impedance on a source with a very low output impedance, but my other sources are "normal" as well.

Hi Wapo
I've built an Arduino-controlled LDR volume control: Sylonex and Arduino preamp. It works great, and the resolution is fine. A drawback is that the circuit is rather complex. An advantage is that carefull matching of the LDRs is not needed.
In my opinion input-resistance of a circuit needs to remain 100% stable to avoid influence on the source before. This means you can calculate the values you need at each position of the pot, taking into account the resistance of the input behind the volume control which comes in parallel with the resistance to ground.
I didn't do this as I have a DCB1 behind the volume, which has a very high input resistance, so I keep the sum of series and ground resistor constant at 50kR. I prefer a relatively high input impedance to avoid upsetting my DAC (a TDA1543 8xparallel).
It's easy to change the total resistance in soft, but I haven't written this yet. It could be better to reduce impedance on a source with a very low output impedance, but my other sources are "normal" as well.

Hello, oenboek:

I have visited your web page in the past and have admired your workmanship.

My goal is to build a very versatile LDR-based volume control but keep it very compact. So far, so good -- the board is 2.5" x 3.8" and everything is there to control any number between one to three LDRs on the shunt side of the pot. The series side will never need more than one LDR. I have a jumper option to run the board either as stereo (1 balance, 1 volume pot) or as dual mono.

Even with the relatively inefficient versions of the LDRs (50 ohms at 10 milliamps) paralleling three devices will allow a minimum resistance of 17 ohms at 10 milliamps. If you select out premium LDR devices, you can achieve 40 ohms with a single device at 10 milliamps and even better if you choose to drive the LDRs at 20 milliamps. There are diminishing returns at the low end -- slightly less resistance requires a lot more current, and I prefer to stay at 10 milliamps with a guaranteed 50 ohms for one device or less with paralleled devices.

The question of what ranges to use for the R1 and R2 is turning out to be a moot point because I'm realizing that my calibration system will allow anyone to select any value of R1 and any value of R2 in any combination. There's no need to plan ahead and set the software up any particular way.

I am now 98% sure that I can deliver a system which will allow the end user to select any combination of R1 and R2 values and calibrate the system at home in about (I'm guessing) 15 minutes. I have finished the drive control code and the beginnings of the calibration code and I've used less than 300 bytes out of an available 4096 bytes, so I'm no longer worried about running out of space to permit both the calibration and the operation code to be installed simultaneously on the PIC.

It sounds like you consider it more important to keep the input impedance steady than anything else? I thought that keeping the output impedance low to benefit the low input impedance of a solid-state amplifier would be more important. I guess it will depend on the source output impedance and the amplifier input impedance, so it's a good thing that the R1 and R2 values will be user selectable.

I note that both you and Tortuga Audio have selected high values for your pots at 50K ohms. Is that because you choose that value to match your other equipment, or are you constrained by the accuracy of your control system? I would have thought that a lower value of pot would be more universally desirable.

I have just started my own passive volume control and come across this thread.
I have been considering relays and resistors, linearity should be perfect and matching between channels extremely good. The only downside I can see is 8 relays (127dB range @ 0.5dB steps) which can cost a few quid each.

This idea of light control looks interesting but Isn't voltage coefficient going to make this quite an imperfect attenuator for high fidelity?
Also wont matching, temperature and current accuracy / noise inhibit performance over fixed R's?

I'm curious to know how that circuit performs. May I ask -- what resistance range does your "pot" cover, and at what supply voltage?

What is the resistance stability at the high end of the range? I'm curious to know how much the resistance wanders at, say, 10K. I have found that this measurement -- resistance wander at 10K -- is probably the most accurate way to judge the effectiveness of a control circuit.

I have been satisfied with a range of +/- 100 ohms at 10K which is 2% and that's good, but I think it's possible to do better, maybe 1%. Current wise, +/- 100 ohms at 10K constitutes a current variation of about +/- .002ma around a base value of .010ma.